Findings are described in five patients who at necropsy were found to have origin of the left main coronary artery from the right sinus of Valsalva and coursing of the anomalously arising artery between aorta and pulmonary trunk to reach the left side of the heart. Three of the five patients were boys and died suddenly at ages 13, 14 and 19 years, respectively: two of them had had one or more episodes of syncope and the third had an abnormal electrocardiogram. The fourth patient, a 64-year old woman, died of chronic congestive heart failure 1 year after an acute myocardial infarction. She had insignificant coronary atherosclerosis. The fifth patient, an 81-year-old man, died of chronic alcoholism, having been free of symptoms of cardiac dysfunction during life. Additionally, clinical and necropsy findings are summarized in 38 previously reported necropsy patients with the coronary anomaly. Of these 38 (34 male [89%]), 23 (61%) died suddenly in the first two decades of life; death in 6 others (16%) appears to have been related to coronary atherosclerosis and 9 patients (24%) died from non-coronary causes. Thus, this anomaly is life-threatening. Why it frequently causes fatal cardiac arrest in some young individuals and allows a normal life span in others remains unclear.